Replace Twisted with aiohttp and native asyncio

[sandhose]

Summary

• Replace Twisted's HTTP server (Resource/Site/NOT_DONE_YET) with aiohttp.web
• Replace Twisted's HTTP client (ProxyAgent/readBody/FileBodyProducer) with aiohttp.ClientSession
• Replace Twisted reactor with asyncio.run() + aiohttp.web.AppRunner
• Replace DeferredSemaphore with asyncio.Semaphore
• Use aioapns native proxy_host/proxy_port instead of ProxyingEventLoopWrapper
• Migrate tests from twisted.trial to pytest + pytest-asyncio + pytest-aiohttp
• Add Pushkin.close() lifecycle method for proper session cleanup on shutdown
• Add ApnsPushkin.close() to shut down the aioapns connection pool
• Harden shutdown: wrap individual pushkin.close() in try/except
• Fix PUSHGATEWAY_HTTP_RESPONSES_COUNTER to count 400 responses (early returns)
• Delete Twisted proxy agent, CONNECT protocol, TLS context factory, and all related test helpers
• Clean up stale mypy.ini sections referencing deleted files
• Use CIMultiDict for DummyResponse.headers so .getall() works correctly

Known limitations

• os.environ["HTTPS_PROXY"] is set as a global side-effect for the Google Auth session proxy. Pre-existing pattern, carried over.
• No explicit SIGTERM handler — relies on asyncio.run() default behavior. May need attention for container deployments.
• Proxy unit tests deleted with no replacement (integration test via Docker compose remains).
• test_overlong_requests_are_rejected deleted — client_max_size enforces the limit but is not tested.

Test plan

• uv run pytest tests/ — 55 tests pass
• CI green: linting, type checking, unit tests, Docker build + proxy check

Part 3 of 7 in the repository modernisation series. Builds on #453.

[reivilibre]

Something I wound up with when trying to pytestify Synapse:

$ cat tests/conftest.py
import os
import tempfile
from collections.abc import Iterator

import pytest


@pytest.fixture(scope="session", autouse=True)
def _run_tests_in_temporary_working_directory() -> Iterator[None]:
    """Run pytest from a temporary working directory.

    The test suite contains many trial-style tests which write temp files relative
    to the current working directory. Running from a temporary directory keeps the
    repository worktree clean when running under pytest.
    """

    old_cwd = os.getcwd()
    with tempfile.TemporaryDirectory(prefix="synapse-pytest-") as tempdir:
        os.chdir(tempdir)
        try:
            yield
        finally:
            os.chdir(old_cwd)

I think it might be as elegant as it gets and also cleans up the dir?